Carbon Distribution in the Stiliwater Complex and Evolution of Vapor During Crystallization of Stillwater and Bushveld Magmas

The occurrence and distribution of carbon in the StillwaterComplex have been investigated. In mineralized troctolite andassociated rocks of olivine-bearing zone I (OB I), carbon ispresent as graphitic material and calcite. The assemblage forsterite-antigorite-calcite-graphiteand the petro graphic relations indicate equilibration of thecarbon-rich phases during serpentinization. Typical OB I troctolitecontains 500–1100 ppm wt. carbon, 40–70% of whichis in calcite, whereas troctolite from higher stratigraphicpositions generally contains <400 ppm carbon. Due to themetamorphism, it is not possible to deduce the extent to whichenrichment of carbon in the ore zone is inherited from magmaticprocesses. In contrast, there is good evidence for magmaticgraphite in parts of the Bushveld Complex. The C-O-H-Cl system has been investigated for conditions ofStillwater and Bushveld crystalliz ation. In alkali-poor fluidsover a wide range of igneous and metamorphic conditions, theimportant chlorine species are HCl and CH₃Cl The addition ofchlorine to a C-O-H fluid in equilibrium with graphite leadsto a quantitative increase in HCl+CH₃Cl and corresponding decreasein H₂O contents, and, when Cl/H exceeds 1, to a CO₂+CO-richfluid with little H₂O Similarly, in more reduced fluids, CH₄contents are depressed by the formation of CH₃Cl. From consideration of volatile solubilities and abundances inmafic magmas and the nature of the C-O-H-Cl system, it is hypothesizedthat the first fluid to exsolve from Bushveld and Stillwaterintercumulus melt was composed of a mixture of CO₂ CO, and HClwith minor amounts of sulfur species and H₂O A model is developedfor the evolution of such a fluid with cooling. The model assumesthat graphite began to precipitate from the fluid at supersolidustemperature and that the system cooled down a T-f_o2 path parallelto and >2 log units below that of the Ni-NiO oxygen buffer.Upon the appearance of graphite, the fluid evolved to a morehydrogen-rich composition by graphite precipitation and lossof oxygen to the surrounding silicate-oxide assemblage. Coolingof fluid to 25?C below the first appearance of graphite resultedin reduction in the fluid mass by >70%, thus concentratingchlorine, sulfur and other residual species in the intercumulusfluid and melt. The model explains the presence of chlor-apatiteand the enrichment of graphite in the Bushveld Critical Zoneand predicts that chlor-apatite-bearing Stillwater rocks weresimilarly enriched in graphite during crystallization.